Migrate historic prometheus data to new prometheus instance

Per a conversation in slack with @tmaczukin

Since https://dev.gitlab.org/cookbooks/chef-repo/merge_requests/1729 , Prometheus on azure is no longer receiving metrics. Instead, they are going to Prometheus on prometheus-app-01.

In the Pages case, metrics in this new datasource start around 2018-02-22, whereas in the former, they seem to start 1-2 months earlier.

Is this historic metrics data valuable enough for us to want to import it into the new metrics server? In my case, I was interested in comparing April performance to March performance, but I can see people wanting to verify a trend across multiple months.

Is it easy / possible to pull the older datapoints into the new datastore? /cc @bjk-gitlab